Pipes in Angular2

This blog explain Pipes in Angular2

In Angular2, Filters are known as Pipes. Those who are not aware with AngularJS, Pipes transform the value in view, e.g.

Uppercase/Lowercase Pipe:
```
My Name in toString Form : {{ name }} 
My Name in Uppercase : {{ name | uppercase }} 
My Name in Lowercase : {{ name | lowercase }} 
```
First line will print the name value as it is, but second line will print the name in the Upper Case while third line will print the name in the Lower Case.

Property name will not change, it would be as it is.
Date Pipe:
```
My DOB in toString Form : {{ birthday }} 
My DOM in Readable Form : {{ birthday | date }} 
```
First line will print full date of birth in local time zone(will call toString() on Date), i.e. Sat Oct 01 1988 00:00:00 GMT+0530 (IST), but second line will print the birthday in more simple format i.e. Oct 1, 1988.

We can pass parameters to a pipe, e.g.
```
My DOM in Custom Form : {{ birthday | date : 'dd/MM/yyyy' }} 
```
In the above line we are providing the date format in which date should be displayed.
JSON Pipe:
```
Object without JSON Pipe : {{ userObject }} 
Object with JSON Pipe : {{ userObject | json }} <h3>Async Pipe:</h3>
```
First line will print [object Object](userObject.toString() returns [object Object]), but second line will print { "name": "Amit Thakkar", "age": 27 }(JSON.stringify(userObject) return { "name": "Amit Thakkar", "age": 27 }). So json pipe will display object in json/readable form.
Async Pipe
```
Async Value without Pipe : {{ age }} 
Async Object without async Pipe : {{ asyncValue$ }} 
Async Object with async Pipe : {{ asyncValue$ | async }} 
```
First line is having a value, which gets initialized asynchronously, and prints the value on view whenever gets initialized. while in the second line we have promise / observable which prints [object Object] instead of value while on third line we are using async pipe so it will print value whenever promise / observable gets resolved.

We can chain multiple Pipes one after one. So output of one pipe will be input for second pipe and so on. Output of last pipe will be displayed on view e.g.

Chaining Pipe result : <strong> {{ birthday | date : 'MMM dd, yyyy' | uppercase }} </strong>

Custom Pipes

We can create Custom Pipes in Angular2, although Angular2 has provided all basic Pipes e.g. Uppercase, Lowercase, Date, JSON, Async etc. But sometimes we have to create custom pipes e.g. range pipe:

| <span *ngFor="let value of (1 | range : 10)">{{value}} | </span>

Here we are repeating the span 10 times, range pipe will create the array of values from 1(min value) to 10(max value), and *ngFor will repeat the span 10 times with value 1 to 10.

Lets see the Range Pipe code:

import {Pipe, PipeTransform} from '@angular/core';

@Pipe({name: 'range'})
export class RangePipe implements PipeTransform {
    transform(minValue:number, maxValue:number):number[] {
        let range = [];
        for (let index = minValue; index <= maxValue; index++) {
            range.push(index);
        }
        return range;
    }
}

So you can see in the above code, implementing the custom pipe in angular2 is quite easy.

Import the Pipe and PipeTransform class from @angular/code.
Provide Pipe name in @Pipe annotation.
Implement transform method of PipeTransform.

transform method will contain all the logic for transforming the input.

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